A suspension comprising finely divided particles of an insoluble substance dispersed in a medium is called a colloidal suspension. A colloidal suspension is characterized by its viscous fluidity property. It is also understood that as the fineness of suspended particles increases or the number or amount of the fine particles increases, the suspension loses its fluidity to show a so-called gelled state. That is, as the degree of fineness or the particle concentration becomes greater, the stability and viscosity of the colloidal suspension are correspondingly increased. A colloidal suspension whose stability or viscosity is sufficiently increased so as to lose fluidity is designated a gel.
It has heretofore been appreciated that a finely divided suspension comprised of a cellulosic material as the insoluble substance can also behave as a colloidal suspension or a gel. In this case, as well, properties of a colloidal suspension or a gel greatly depend on the size, size distribution, and concentration of the suspended insoluble particles of the cellulosic material.
However, it is difficult to finely divide cellulosic material and the state-of-the-art suspensions of a cellulosic material possess only a relatively low degree of fineness despite all the prior efforts made to more extensively comminute the cellulosic material.
The inventors of the present invention have ground wood pulp fibers by using conventional techniques such as a dry ball mill to the full extent possible. Notwithstanding a grinding operation of over 16 hours, it was not possible to obtain particles having a 50% cumulative volume diameter of 12 .mu.m or less. The term "50% cumulative volume diameter" used herein means a diameter of particles, assuming the particles as being spherical, at which the volume of particles cumulated in order from small ones to large ones reaches 50% of the total volume of the whole particles.
On the other hand, conventional finely divided cellulosic materials include, for example, a microcrystalline cellulose which is a particulate cellulose mainly comprising crystallite aggregates obtained by removing amorphous regions of a cellulose raw material, such as a wood pulp and cotton linters, by hydrolytic degradation with a mineral acid. The microcrystalline cellulose generally comprises coarse particles predominantly having a size of from 15 to 40 .mu.m. A suspension obtained by stirring the microcrystalline cellulose in water exhibits viscous fluidity, dispersion stability, and gelforming properties. These properties are attributed to further size reduction of the above-described coarse particles originally sized at from 15 to 40 .mu.m. According to JP-B-40-26274, page 2, right column, lines 6-10 (the term "JP-B" as used herein means an "examined published Japanese patent application"), these properties are considered as attributed to an increase in the population of crystallites of 1 .mu.m or smaller in size. In actuality, however, the proportion represented by crystallites of a size of 1 .mu.m or smaller is no more than 2% by weight, with the remainder of the suspended particles still comprising the coarse larger-sized particles. For the purpose of confirming this fact, the present inventors measured a particle size of a suspension which was prepared by treating a mixture of a microcrystalline cellulose powder and water having a prescribed solid content in a homo-mixer at a rate of 10000 rpm for 5 minutes. The obtained suspended particles did not have a 50% cumulative volume diameter of 14 .mu.m or less, and a total volume of those having 3 .mu.m or less was not more than 6% of the total volume of the whole suspended particles, and a total volume of those having 1 .mu.m or less was not more than 1%. (The above volume ratio is hereinafter referred to as "cumulative volume ratio".) That is, none of the thus obtained suspensions was satisfactory in terms of viscosity and stability.
In order to improve these cellulosic particle size properties, it has been proposed to treat a microcrystalline cellulose suspension by means of a high pressure homogenizer as disclosed in JP-B-62-30220. The high pressure homogenization as proposed comprises repetition of high energy application for a short period of time by passing the suspension through a small diameter orifice at a high speed under a high shear force while giving a pressure difference of at least 200 kg/cm.sup.2 and then striking the spouted suspension against a wall surface to drastically reduce the suspension steam speed. JP-B-62-30220 reference describes that a stable and highly viscous suspension can be obtained by the high pressure homogenization but does not disclose the degree of fineness of the resulting particles. As a result of careful experiments duplicating the high pressure homogenization disclosed in JP-B-62-30220, the present inventors have confirmed that the particles which are the most finely divided by the high pressure homogenization still do not have a 50% cumulative volume diameter reduced to 7 .mu.m or less; with the cumulative volume ratio of particles of 3 .mu.m or less being 20.8% at the most, and that of particles of 1 .mu.m or less being 3.9% at the most. It appears that crystallites which are aggregating with a relatively weak force can be degraded at a relatively high efficiency by a cavitation effect of high pressure homogenization whereby the particles are reduced in size to increase stability and viscosity. However, the effect of degradation or comminution is not exerted on those crystallites which are more densely aggregating with a strong force, and size reduction does not significantly occur for these crystallites during high pressure homogenization.
Further, JP-A-56-100801 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") which corresponds to GB Patent 2,066,145 discloses fine fibrous cellulose (microfibril cellulose) obtained by treating an aqueous suspension of a pulp in a high pressure homogenizer. This suspension is an aggregate of fibrillated fibers which are not divided into fine particles and comprises coarse particles having a 50% cumulative volume diameter of 95 .mu.m or greater. The fibers have a length to diameter (L/D) ratio of 100 or more as shown in FIG. 1 and, therefore, feel very rough to the touch and can be easily twisted by fingers into a string. Further, these fibers are unsuitable as edible fibers because when taken in one's mouth, they feel rough to the mouth and tongue and are easily entangled with each other to form masses of rather unpleasant palatability. Further, the microfibril cellulose cannot be formulated into a high concentration suspension, generally providing a suspension having a concentration of only 2% by weight. This is one of drawbacks of microfibril cellulose as pointed out in Shokuhin to Kaoaku, Issue of Nov., 1983, p. 51, col. 4, lines 26-29.
JP-B-2-12494 discloses a process for producing finely divided cellulose particles, in which cellulose fibers from which lignin has been substantially removed by a lignin removal treatment is sealed in a pressure container, heated in a hydrated state under pressure, and rapidly and instantaneously spouted into a reservoir under normal pressure. According to this process, cellulose fibers can be finely divided by the vaporizing force resulting from abrupt pressure release and mechanical impact (and/or grinding) among fibers and between the fibers and the wall surface resulting from rapid spouting. The process, however, cannot provide particles having a 50% cumulative volume diameter of not more than 11 .mu.m. Besides, since the cellulose fibers are exposed to high temperatures of 200.degree. C. or more, they undergo considerable color change to black brown hues and also undergo denaturation.
JP-A-1-293144 discloses a process for finely dividing a wood meal in a wet process by using dimethylformamide, toluene, and the like, as a medium. It describes the resulting particles as having a diameter as small as 6 .mu.m, but this is in reference to a diameter of the shorter axis. The particle size as referred to in the present invention is a value measured with a laser-beam diffraction particle size distribution measuring apparatus. A finely divided cellulosic material having a 50% cumulative volume diameter of 6 .mu.m has an L/D ratio of from 5 to 10, from which the diameter of the shorter axis of these columnar particles can be calculared to be in the range of 3.1 to 2.4 .mu.m. Also, when observed under an electron microscope, almost all of the particles have a shorter axis diameter of not more than 3 .mu.m. Particles finely divided by the process of the present invention to have a 50% cumulative volume diameter of, for example, 0.37 .mu.m, have a shorter diameter of 0.1 .mu.m under electron microscopic observation. That is, the particles obtained by the process of JP-B-1-293144 are far greater in size than those of the present invention.
As described above, despite many previous efforts to obtain a suspension of fine particles of a cellulosic material exhibiting properties of a colloid or a gel, a smooth, stable, and thick suspension has not yet been obtained due to difficulty of finely dividing a cellulosic material.